1. Field of the Invention
This invention relates to simulation of electronic systems, and more particularly, to the simulation of power and ground planes.
2. Description of the Related Art
Designing a power distribution system often times includes the performing of simulations of both the power distribution system itself and the circuit(s) to which it will supply power. Such simulations may be conducted using various software tools and models (e.g., SPICE models) of the power distribution system. The simulations may be used to verify the operational characteristics of the power distribution system, and may also provide information useful in altering a design to meet specifications.
Creating a model of a power distribution system includes the modeling of power and reference (e.g., ground) planes. One commonly used method of modeling power and ground planes includes creating grids of square or rectangular cells, where each cell may be modeled as transmission line segments. FIG. 1A is a drawing of an exemplary grid of cells. In this particular example, each cell includes four nodes which are connected to other nodes by transmission line segments in order to form cells.
For power/ground planes that are square in shape and continuous (e.g., no apertures in the plane), this method may work well by allowing the grid to easily approximate the size and shape of the plane being modeled. However, this method may have drawbacks for planes with apertures or irregular shapes.
FIG. 1B is a drawing of a uniform shaped grid superimposed on an irregularly shaped plane. In this particular example, grid 7 is superimposed on plane 5. While grid 7 has a uniform rectangular shape, plane 5 has an irregular shape which includes apertures in the plane. Grid 7 is a 28×20 grid, and thus includes 560 cells. Thus, in this instance there are 91 cells that are completely outside of the plane shape.
In software tools such as SPICE, an increase in the number of nodes in a grid (which corresponds directly to the number of cells in the grid) may cause a sharp increase in the amount of time required to perform the various simulations. Furthermore, the extra time used to perform the simulation may yield no additional useful output.
Another hazard of using this method is that the accuracy of the simulation may be compromised. In particular, using a uniform shaped grid to approximate an irregular shaped plane may cause resonances to be characterized incorrectly. Some resonances may be dependent upon standing wave patterns which are determined by the actual shape of the plane, including any apertures. As such, both the location and frequency of these resonances may not be accurate with respect to the actual plane being simulated. Accurate characterization of resonances may be especially important in the design of power distribution systems for determining locations where decoupling capacitors are to be placed, as well as determining the electrical characteristics of the capacitors to be used. Thus, if resonances are improperly characterized, it may not be possible to meet target impedance requirements for the power distribution system.